Lighting applications can be implemented with a combination of predetermined lighting devices operating at predetermined light intensity levels. For example, a residential lighting application may require a variety of lighting scenarios, or “scenes.” A first scene may be needed for when the residents are at home and active within the house. In such a scene, lights at various locations may be illuminated with full intensity to enable safe movement within the house. A second scene may be needed for when the residents are out of the house. For example, selected outdoor and indoor lights may be illuminated at various intensity levels for security or other reasons. Likewise, additional scenes may be configured for when the residents are on vacation, entertaining, or for any other type of activity. As the number of lighting devices and/or scenes increases, it becomes more convenient to control the lighting devices from a central location, rather than by controlling each lighting device individually.
Various systems exist that allow for the remote control of lighting devices in a lighting application. Wireless lighting control is frequently used in residential and commercial applications because of the ease and low cost of installation as compared to wired systems. Wired system have numerous shortcomings that result from the need to hard-wire lighting control devices within a lighting application. For example, retrofitting an existing building to accommodate a wired system may involve routing wires through walls and other structures, installing cable trays or conduit, and/or running wire through existing conduit. If a building into which the wired system will be installed is still in the planning phases, then accommodations for the wires need be made in the design plans for the building if the above noted retrofitting issues are to be avoided. In either case, the planning for and installation of a wired system requires effort that increases costs.
In contrast, a wireless system is often a more economical choice than hardwired lighting control systems because the need to install and connect wiring, which is particularly problematic in existing buildings, is largely eliminated. Instead of having to plan for the installation of lighting control devices during the design of a building, or having to retrofit an existing building, the owner or operator of the building may simply place a lighting control device wherever such device is desired. Such a device may be battery powered or may simply be connected to a power outlet. The cost savings of wireless systems is especially noticeable in older, existing buildings that would otherwise require complicated and/or cumbersome retrofitting. Wireless systems are also a preferred choice for home applications, as such applications are typically more cost-sensitive than commercial applications.
One way to implement a wireless lighting control system having wireless lighting control devices is to enable such devices to communicate with each other by way of Radio Frequency (RF) transmissions. An example of such a RF system is the RadioRA® system manufactured by Lutron Electronics Co., of Coopersburg, Pa. In the RadioRA® protocol, all devices within a subnet—where a subnet is an individual RadioRA® system—operate on the same frequency. The use of a single frequency may be made to avoid interference with other devices within the building, to comply with FCC regulations, to reduce costs and the like. As a result, however, it is possible that the devices within a subnet may interfere with each other as a result of transmitting at the same time on the same frequency. In addition, in existing RF lighting control systems there is a limitation as to the number of devices that can be controlled on a single network. Too great a number of devices will run afoul of FCC regulations because such regulations permit transmissions of only a certain length of time on a particular frequency. Current systems, such as RadioRA®, allow for a maximum of 32 devices to be controlled.
In some applications it is necessary to use more lighting control devices than a single subnet is capable of controlling. Therefore, a second subnet may be needed to control all of the desired devices. It will be appreciated that placing two wireless lighting control systems in close proximity to each other when both are operating on the same frequency poses serious problems, particularly when a lighting scene involves both subnets. Specifically, it is possible that the individual subnets will communicate simultaneously and therefore would interfere with each other by causing messages to collide and by unnecessarily populating the RF. While the chances of interference within one subnet may be small because of the relatively short RF transmission times typically used within a single subnet, in multiple subnet scenarios the RF transmission times increase because of the greater number of devices that must receive and send RF transmissions.
For example, when two unrelated subnets are located in close proximity, each subnet runs a risk of interfering with the other. However, because each subnet is unrelated, the timing of lighting events—such as a scene—in each subnet will only occur at the same time as a coincidence. In contrast, when two or more subnets are functionally grouped together, a lighting scene that involves more than one subnet deliberately causes each effected subnet to communicate at the same time. As a result, in multiple subnet systems, the RF transmission times increase to the point that interference is likely.
Accordingly, what is needed is a method for increasing the number of devices that can be controlled by a lighting control network that uses a single RF. More particularly, what is needed is a method of linking multiple subnets that can co-exist as individual entities operating on the same RF as well as interact and communicate globally with each other without data collisions. Even more particularly, what is needed is a method for initiating programmable lighting events involving multiple subnets by way of a central control.